Circuit Breakers and the CBMU

The overview planted a counter-intuitive fact and left it for later: the A330 has no circuit-breaker panel in the cockpit. Every breaker lives in the avionics compartment or the bulk cargo hold, beyond a pilot's reach. So everything a crew ever perceives about a tripped breaker arrives through one box — the Circuit Breaker Monitoring Unit (CBMU) — which reports breaker status to the ECAM C/B page. The design deliberately splits seeing from touching: you can see which breaker has opened, but you cannot get to it. That "cannot get to it" is itself part of the safety design, the physical backstop behind the re-engagement discipline of §4.

This article owns the whole breaker-awareness chain: where the breakers are and which ones the CBMU watches, how the CBMU senses a breaker and decides its state, the C/B TRIPPED warning and its flight-phase window, the re-engagement rules a pilot actually follows, and the C/B page with its two distinct "unavailable" messages. The RCCB component itself and its TRIP signal are treated in AC Distribution and Busbars; for the crew, an RCCB trip and a plain C/B trip land on the same ECAM list and follow the same handling logic.

1. Where the breakers live, and who watches them

FCOM gives the one-line system intent, and the AMM gives the availability statement:

"The Circuit Breaker Monitoring Unit (CBMU) monitors the aircraft circuit-breaker status. It detects the circuit breakers which are open and transmits their identification through an ARINC 429 datalink to the Electronic Centralized Aircraft Monitoring (ECAM) System Display (SD). The CBMU operates when it is energized. It is available on ground and in flight at all times."

Per AMM 24-53-00 §1, rev 01 APR 26. The breakers themselves are scattered across two zones, and only some of them fall inside the monitored circle:

The non-monitored panels are filed under a precise AMM heading:

"Non-Monitored Circuits Breakers on the Panels Located in the Cabin"

Per AMM 24-53-00 §5.D(3), rev 01 APR 26. Read it carefully: these six panels are purely in the cabin, not the cargo hold — the breakers that are in the cargo hold (the APU control box, 5000VE) are precisely the monitored ones, the opposite direction. The AMM's only qualifier for the unwatched panels is "cabin + non-monitored". FCOM, separately, frames the same breakers by function:

"All Circuit Breakers are monitored except commercial Circuit Breakers."

Per FCOM DSC-24-20, rev 14 JAN 26. So "commercial" is the FCOM word (it does not appear in AMM 24-53 at all); "located in the cabin" is the AMM word. Both describe the same set.

[!warning]- A "coffee maker is dead" cabin report with a clean C/B page is not a contradiction

If a cabin-crew member reports that a galley or cabin-service item has lost power while the ECAM C/B page shows NO C/B PULLED, the two facts do not conflict — those cabin panels (5001–5060VE) sit outside the CBMU monitoring circle, so a trip there never reaches the ECAM. Log it for maintenance; it is not a CBMU fault.

A point on the hardware itself: the CBMU is FIN 1XD, in the avionics compartment on panel 120, a single unit. Do not confuse it with the "CBMU disk" — the AMM notes the disk "is installed in the cockpit, in a box immediately behind the pilot seat" (per AMM 24-53-00 §2). The disk is the configuration medium, not the computer.

2. Inside the CBMU — how it sees a breaker

Think of the CBMU as the remote inspector of a building's main switch room: every breaker is locked away downstairs (the avionics bay), each tenant (the crew) has only a screen upstairs (the C/B page), and the inspector watches a reverse-logic "sentinel contact" on every breaker — when a sentinel falls silent it reports "status unknown" rather than falsely claiming all is well.

   monitored C/B auxiliary contact            reverse logic:
   (one per breaker)                          main contacts CLOSED -> auxiliary contact OPEN
            │
            │  wiring matrix    side 1 : 26 groups of 30 C/B (max)
            ▼                   side 2 : 16 groups of 30 C/B (max)
   ┌──────────────────────────────────────────────────────────────┐
   │  CBMU   (FIN 1XD, avionics compartment, panel 120)            │
   │   · CPU reads contact statuses in batches of 8                │
   │   · three-state verdict :  open / closed / unknown            │
   │   · NVM database :  21 ASCII characters + FIN per C/B,        │
   │                     1168 position indexes                     │
   │   · power :  ONE 28 VDC source only  —  busbar 101PP          │
   └─────────────────┬───────────────────────────┬────────────────┘
                     │ ARINC 429                  │ BITE
                     ▼                            ▼
          ECAM   DMC ─► SD C/B page              CMC (maintenance)
                 EWD ─► warning

2.1 The three-state verdict and the reverse-logic contact

The CBMU's core monitoring function reads each breaker's auxiliary contact and returns one of three states:

"Permanently, the CBMU monitors the circuit breaker auxiliary contact status. The Central Processing Unit (CPU) receives circuit breaker contact statuses by batch of 8. This function determines after analysis the status of the circuit breaker which can be: - open, - closed, - or unknown (failure case: if neither open nor closed can be confirmed)."

Per AMM 24-53-00 §6.C(1), rev 01 APR 26. The wiring behind it is deliberately inverted:

"The CBMU individually monitors the circuit breakers through a wiring matrix. The arrangement of the wiring matrix is for: - side 1: 26 groups of 30 circuit breakers maximum, - side 2: 16 groups of 30 circuit breakers maximum. To do this the CBMU uses the auxiliary contact of each circuit breaker which is open when the main contacts are closed."

Per AMM 24-53-00 §6.C(2), rev 01 APR 26. Two things matter here. First, "side 1 / side 2" is not just matrix size — it is a panel split. From the configuration cross-reference table (CORRESP.TAB), side 1 carries the 710VU group plus 721VU, plus 5000VE and 742VU; side 2 carries the 710VU group plus 722VU. Second, the reverse logic makes broken monitoring self-exposing: because a healthy closed main contact corresponds to an open auxiliary contact, a broken sense line reads a state that contradicts the normal one — so it falls into unknown rather than masquerading as a false "closed". A dead sentinel announces itself.

2.2 The identity store and the two breaker categories

Each breaker carries an identity and a category in non-volatile memory:

"The CBMU has a Non Volatile Memory (NVM) that contains the data related to all the monitored circuit breakers. Twenty-one ASCII alphanumeric characters identify each circuit breaker. There are two categories of circuit breakers: - Category 1 (no change is possible): the circuit breakers that are always fully functional (status: closed) - Category 2 (can be changed): the circuit breakers in the media database can have two statuses that are related to the aircraft configuration: . Closed for fully functional circuit-breakers . Open for pulled and secured circuit breakers."

Per AMM 24-53-00 §6.C(3), rev 01 APR 26. The store spans 1168 positions (per AMM 24-53-00 §6.B: "The number of circuit breakers which can be monitored is: - 1168 circuit breakers and 16 provisions."). The category split is the mechanism behind "a deliberately pulled breaker does not nag you forever": when maintenance pulls and secures a breaker (under the MEL or a troubleshooting task) they use the PDL (Portable Data Loader) to set it Category 2 / open in the database — from then on the CBMU treats it as a registered opening, not an unplanned trip, and raises no warning for it. The alert is aimed at the unexpected open, never at the on-the-books one.

2.3 The APU control box — monitored in parallel

The breakers on the APU control box (5000VE) are handled as a single combined entry, which is why a trip there shows up as one line:

"These circuit breakers are monitored in parallel. The CBMU sees them as one single circuit breaker without a FIN (this data field are empty) and with "APU" for the label, 10 for the status, 0 for the category and 5000VU for the position."

Per AMM 24-53-00 §6.C(3)b, rev 01 APR 26. So however many breakers sit on that panel, the C/B page presents them as a single "APU" item — expect one line, not a list, when that box is involved.

2.4 The four functions, and the single-source power gotcha

The CBMU performs four functions (per AMM 24-53-00 §6.C): breaker monitoring, auxiliary-contact status acquisition, breaker identification and transmission, and — optional equipment only — monitoring of breakers that are deliberately pulled and safetied (it raises a maintenance message if such a breaker is found not pulled). The first three are the everyday job; the fourth is a configuration-engineering nicety, mentioned for boundary only.

The power arrangement carries a trap a pilot must hold:

"The CBMU is supplied by only one 28 VDC power source, from busbar 101PP. Consequently, in some emergency configurations, the CBMU is not supplied."

Per AMM 24-53-00 §4.A, rev 01 APR 26. In some emergency configurations the whole of breaker monitoring goes dark. That is a sound trade — monitoring is a luxury, and an emergency configuration counts every watt — but it means that in EMER CONFIG the C/B page may read NOT AVAIL, and you should not lean on it for diagnosis. (101PP is a sub-bus of the 28 VDC network; the precise list of emergency configurations in which the CBMU drops is summarised by the AMM only as "some emergency configurations" and is not extrapolated here.)

3. The C/B TRIPPED warning and its flight-phase window

When the CBMU detects one or more open breakers, the warning it raises is gated to specific flight phases:

"Up to the second engine start, above 1500 ft (after TAKE OFF) and 800 ft (in APPROACH) and below 80 kts (after LANDING), the crew is warned by the MASTER CAUT light and the single chime when the CBMU has detected one or several open circuit breakers."

Per AMM 24-53-00 §7.C(1), rev 01 APR 26. The right way to read that string is to read its complement — the phases in which the warning is suppressed:

   ARMED  (MASTER CAUT + single chime if a breaker opens):
     ├─ up to the 2nd engine start
     ├─ above 1500 ft after TAKE OFF
     ├─ above 800 ft in APPROACH
     └─ below 80 kt after LANDING

   INHIBITED  (the complement — the high-workload phases):
     ├─ 2nd engine start ──► 1500 ft     (take-off roll + initial climb)
     └─ 800 ft ──► 80 kt                 (short final + landing roll)

This is ordinary ECAM flight-phase inhibition (ATA 31): a single tripped breaker is not allowed to interrupt you during the take-off roll or the landing roll. If a breaker opens on the take-off roll you will not be warned then — the caution waits until you climb through 1500 ft. The system is managing your attention, not missing the event. The warning is accompanied by an EWD message and the automatic call of the C/B-related page; to see the detail, press the C/B key on the ECAM control panel to bring up the SD list (functional designation + position + FIN).

The CBMU keeps the list ordered most-recent-first. Per AMM 24-53-00 §7.B: once all breakers have been checked, the CBMU transmits the count of open breakers and their identification list, and "if the number of open circuit breakers increases between two acquisition cycles, the identification of the most recently opened (pulled or tripped) circuit breakers are placed at the beginning of the identification list." The newest trip is always on top.

4. Re-engagement discipline — the FCOM rule, not the AMM prohibition

The AMM and the FCOM speak to re-engagement at two different layers, and a pilot must follow the FCOM one. The AMM, at the maintenance layer, writes a blunt prohibition:

"- in flight, it is not permitted to close a tripped circuit breakers. - on ground, you must not close a tripped circuit breaker without trouble shooting of the related system."

Per AMM 24-53-00 §7.C(2), rev 01 APR 26. But the operational authority for the crew is the FCOM/QRH procedure, and it is not an absolute ban — it is a conditional re-engagement with captain's discretion:

"On ground: Do not reengage the circuit breaker (C/B) of the fuel pump(s) of any tank. For all other C/B, if the flight crew coordinates the action with maintenance, the flight crew may reengage a tripped C/B, provided that the cause is identified. In flight: Do not reengage a circuit breaker (C/B), unless the captain judges it necessary to do so for the safe continuation of the flight. Only one reengagement should be attempted."

Per FCOM PRO-ABN-ELEC (ELEC C/B TRIPPED), rev 14 JAN 26. So the rule a pilot carries is:

• In flight — as a rule, do not re-engage; the only exception is when the captain judges it necessary for the safe continuation of the flight, and then only one attempt.
• On ground — for any tank's fuel-pump breaker, never re-engage; for all other breakers, re-engagement is permitted after coordinating with maintenance and once the cause is identified.

The AMM's "never in flight" is maintenance-document wording, refined and superseded by the FCOM crew procedure — do not teach it as a pilot's absolute prohibition.

The reasoning under the layered gates (an integrative reading, not verbatim): a trip means the circuit has already carried an over-current once, so a real fault — a short or a stalled load — is the likely cause. Blindly re-closing sends power straight back into that fault, and the worst outcome is an electrical fire. That is precisely why the FCOM stacks gates — cause identified / single attempt / captain's judgement — rather than issuing a flat "no". The fuel-pump breakers are barred even on the ground because an electrical source inside a fuel tank is the highest ignition concern. And the cockpit's physical layout helps enforce all of this: the breakers are out of reach, so the impulse to "just pop it back in" has nowhere to go.

5. The C/B page and the two unavailability messages

The C/B page has an idle state, a list state, and two distinct "unavailable" states. The idle state, in current FCOM wording, is NO C/B PULLED — not "NORMAL":

"NO C/B PULLED — (1) Displayed in green when no circuit breaker is pulled."

Per FCOM DSC-24-20, rev 14 JAN 26. ("NORMAL" is older AMM §7.C(3) wording; the pilot-facing FCOM term is NO C/B PULLED.) The full set of page states:

Three things to keep straight. First, the FCOM C/B-page description covers only the two operating displays (NO C/B PULLED and the open-breaker list); NOT AVAIL / NO AVAIL are AMM §7.C messages, not FCOM page names. Second, NOT AVAIL and NO AVAIL are two different messages: NOT AVAIL means "the CBMU is faulty", NO AVAIL means "the CBMU is being loaded, ARINC links cut" — do not merge them. Third, the CBMU's own failure is a separate ECAM alert.

When the CBMU itself is lost, the ECAM alert is ELEC C/B MONITOR FAULT:

"This alert triggers when the CBMU is lost."

Per FCOM PRO-ABN-ELEC, rev 14 JAN 26 — handled by "Crew awareness." only.

[!warning]- The two warnings sit at different levels — and the level source differs between manuals

The AMM figure captions tag the two events in the older Class/Level scheme: C/B TRIPPED = Class 1 – Level 2 (a real-fault level, MASTER CAUT + single chime), CBMU fault = Class 2 – Level 1 (a low, monitor-failure level). The current FCOM PRO-ABN-ELEC marks both at L2. For the crew the FCOM level is the reference; the AMM Class/Level belongs to the maintenance classification layer. The practical lesson holds either way: a failed monitor is worth knowing about, but it is never worth interrupting a take-off — and note its own flight-phase window (up to the second engine start, and above 800 ft) per AMM 24-53-00 §7.C(4).

6. Flight-deck scenarios

1. Cruise MASTER CAUT — C/B TRIPPED. Bring up the C/B page, read the list (newest on top), identify the system, and handle its consequences by that system's ECAM/QRH procedure. Re-engage? As a rule no in flight — only if the captain judges it necessary for safe continuation, and then one attempt only (§4). The breaker is out of reach anyway, which usefully suppresses the impulse.
2. A breaker trips on the take-off roll. You receive no warning then (inhibited window) — the caution arrives only after 1500 ft. The design is managing your attention, not failing to report.
3. Cabin crew report a service item dead, but the C/B page shows NO C/B PULLED. Those cabin panels (5001–5060VE) are outside the monitored circle. Record it for maintenance; it is not a CBMU fault.
4. EMER CONFIG with the C/B page reading NOT AVAIL. The CBMU's single source (101PP) is unpowered in some emergency configurations — monitoring going dark here is by design, not a new fault. (Distinguish: NOT AVAIL = CBMU faulty; NO AVAIL = CBMU being loaded, ARINC links cut.)
5. A breaker pulled and secured under the MEL. Maintenance set it Category 2 / open in the CBMU database via the PDL, so it sits quietly in the registered state and raises no C/B TRIPPED warning. After flight, cross-check any C/B-page list item against the dispatch paperwork before treating it as a trip.

7. Dispatch view (MEL)

Two dispatch threads run through this article.

A pulled-and-secured breaker does not get its own "C/B" MEL item — it is dispatched under the affected system. The MEL routes a ELEC C/B TRIPPED to "refer to the affected system", i.e. you judge dispatch by the MEL entry for whatever that breaker feeds (a pump, a light, and so on). Setting it Category 2 / open in the CBMU database is only a configuration step to stop the false report — it is not itself the dispatch basis.

The CBMU itself is dispatchable. Per the operator MEL (MI-24-53-01A), the CBMU is Category C (1 installed / 0 required, no placard) and "may be inoperative provided all circuit breakers in the power compartments are checked closed (main power centre and emergency power centre)", with deactivation per AMM 24-53-00-040-801. The logic is clean: the monitor is a luxury, so its loss is acceptable — but the price is losing the eye on the breakers, so before dispatch a human must go into the avionics bay (zone 120) and the bulk cargo compartment (zone 162) and confirm every breaker is closed. "The machine can no longer see" is exchanged for "a person looks once". This is the same dispatch tier as the galley supply (Galley and Commercial Loads): a monitoring-class item, Category C, go — but conditioned on a manual check. The full electrical dispatch picture is in MEL Dispatch View.

Self-test

[!note]- Q1. Where do the breakers live, and which ones does the CBMU watch?

Monitored: the main power centre 715/717/718/721/722/725/735VU (avionics bay, zone 122), the emergency power centre 742/743VU (zone 121), and the APU control box 5000VE (bulk cargo compartment, zone 162 — monitored in parallel, so it shows on the C/B page as a single "APU" item). Outside the circle (not monitored): the cabin panels 5001/5002/5005/5006/5058/5060VE (AMM: "Located in the Cabin" — purely cabin, not cargo). There are no breakers in the cockpit.

[!note]- Q2. How does the CBMU sense a breaker, and what are the three states?

Through each breaker's auxiliary contact (reverse logic: main contacts closed → auxiliary contact open), wired into a matrix (side 1 = 26 groups of 30 max; side 2 = 16 groups of 30 max), with the CPU reading statuses in batches of 8. The verdict is one of three: open / closed / unknown (neither confirmable = an acquisition-chain fault; the reverse logic makes a broken sense line self-exposing rather than falsely "closed"). The NVM holds 1168 position indexes, 21 ASCII characters per breaker. It runs on a single source — busbar 101PP — so in some emergency configurations the CBMU drops out entirely.

[!note]- Q3. What is the C/B TRIPPED warning window, and why is it shaped that way?

The crew is warned (MASTER CAUT + single chime) only up to the second engine start / above 1500 ft after take-off / above 800 ft in approach / below 80 kt after landing. The complement — the take-off roll + initial climb, and short final + landing roll — is suppressed: ECAM flight-phase inhibition keeps a single tripped breaker from interrupting the highest-workload phases.

[!note]- Q4. What is the re-engagement discipline?

The pilot rule is the FCOM one, not the AMM "absolute prohibition". In flight: as a rule do not re-engage, unless the captain judges it necessary for the safe continuation of the flight, and then only one attempt. On ground: never re-engage any tank's fuel-pump breaker; for all others, re-engagement is allowed after coordinating with maintenance and once the cause is identified. The mechanism: a trip means the circuit already over-carried once, so blind re-closing re-feeds a likely fault (worst case, an electrical fire) — hence the stacked gates of cause-identified / single-attempt / captain's discretion. The AMM "never in flight" is maintenance wording, refined by the FCOM crew procedure.

[!note]- Q5. What are the C/B page states, and the CBMU-fault alert?

NO C/B PULLED (green) = nothing open (FCOM term; older AMM "NORMAL"); the list = breakers open (newest on top / designation + position + FIN / max three pages); NOT AVAIL = CBMU faulty; NO AVAIL = CBMU being loaded, ARINC links cut (two different messages). The CBMU's own failure is the ECAM alert ELEC C/B MONITOR FAULT — "This alert triggers when the CBMU is lost", handled by "Crew awareness" (FCOM L2; AMM figure caption Class 2 / Level 1). C/B TRIPPED itself is AMM Class 1 / Level 2.

Key takeaways

References

Per AMM 24-53-00 D/O (CBMU purpose and availability; the monitored/non-monitored panel split with the "Located in the Cabin" heading; the FIN 1XD location and the cockpit "CBMU disk"; the three-state monitoring and the reverse-logic wiring matrix side 1 = 26×30 / side 2 = 16×30; the NVM 21-character identity, 1168 indexes, Category 1/2 and PDL reprogramming; the parallel-monitored "APU" item on 5000VE; the four functions including the optional pulled-and-safetied monitoring; the single 28 VDC source 101PP and emergency-configuration drop-out; the C/B TRIPPED flight-phase window and the most-recent-on-top list; the AMM re-engagement wording; the NORMAL / NOT AVAIL / NO AVAIL messages and the Class/Level figure captions); FCOM DSC-24-20 (NO C/B PULLED green, the "monitored except commercial" qualifier, the overflow symbol and three-page maximum); FCOM PRO-ABN-ELEC (the ELEC C/B TRIPPED re-engagement procedure — ground vs in-flight, captain's discretion, single attempt; the ELEC C/B MONITOR FAULT alert "triggers when the CBMU is lost", Crew awareness, both at L2); the operator MEL (24-53-01A CBMU dispatch — Category C, all power-compartment breakers checked closed; a pulled breaker dispatched under its affected system). The reverse-logic self-exposure reading, the layered re-engagement reasoning, and the "monitor is a luxury, exchange machine-sight for a human check" dispatch reading are integrative syntheses of the above and contain no facts from outside the library.

Independent study material, not an Airbus publication. Refer to current operator FCOM, FCTM, and QRH for operational use.